Building standards for offices, factories, hotels and hospitals usually require the ability to make announcements to all people within the confines of the premises. These announcements are made possible by means of a suitable microphone-amplifier-speaker arrangement. The amplifier is typically operated from the 60 hertz utility mains. If, however, the 60 hertz utility power should fail, the ability to make announcements would be lost. In certain emergency situations, this inability to make announcements could make it impossible to inform and direct those people within the confines of the building in question, resulting in needless injury and loss of life. Recognizing this danger, building standards have increasingly required that the amplifier means be capable of operation from battery power in the event of mains power failure.
At present, two conventional methods are employed to provide an amplifier with the ability to operate in the event of a 60 hertz mains power failure. Both methods use batteries as the back-up power source. One method results in reduced amplifier performance and requires the use of certain undesirable (due to size and cost) components. The other approach requires that in the event of a mains power failure, the amplifier 60 hertz power input be connected to an expensive, separate unit which will convert battery voltage to 60 hertz, 120 vac.
The first method utilizes what is called the "push-pull" circuit configuration. This approach requires that (via some transformer, rectifier, filter means) the 60 hertz mains power be converted to a single positive low voltage (typically 24 vdc) at high current. The transformer, rectifier and filter means which are capable of providing this low voltage at high current are inherently expensive, due to the restrictions imposed by the physics of handling high currents in such electrical devices. As this circuit operates internally at both a single polarity and a low voltage, some transformer means is required to convert the electrical output of the amplifier to a signal which is high voltage and both positive and negative in polarity. This process requires that the output transformer means be an integral component in the amplification process, for without the transformer, there could be no usable electrical output from the amplifier. Due to the critical interaction of the output transformer with the amplifier circuitry, and the physics of the output transformer, sizable quantities of expensive materials are required to obtain any degree of desirable overall amplifier performance, which results in economic inefficiency. For reasonable amplifier operation characteristics, a feedback winding is required. The extra quantities of higher quality material and the additional feedback winding (which is itself difficult to design) contribute adversely to the expense of the push-pull output transformer.
To operate the above "push-pull" amplifier from a source of battery power, all that is required is a simple means of switching the amplifier's low voltage single polarity power supply to the battery. This transfer takes place only in the event of a 60 hertz mains power failure. While the "push-pull" configuration is easy to operate from a backup battery power source, the economic efficiency of this format suffers from: (1) large and expensive power transformers and associated devices (rectifiers and filters), (2) large and expensive output transformers, and (3) poor performance (frequency response and distortion) due to output transformer limitations and interactions. These limitations exist when operating from either battery or mains power.
A second presently employed method of operating an audio power amplifier from a source of battery power in the event of a 60 hertz mains power failure is not limited to any particular amplifier type. This technique relies upon the use of an inverter. The inverter converts DC battery voltage into 120 vac 60 hertz, as could be obtained from the power mains. The inverter is external to the power audio amplifier configuration. The output of this independent battery powered unit will (in the event of a 60 hertz mains power failure) be connected, via a suitable means of switching, to the mains power input of the audio amplifier. This approach allows the use of any type of audio amplifier, making it possible to obtain very high performance (wide frequency response, low distortion) while operating from a source of battery backup power. The problem with this approach is that the cost of the separate inverter unit is typically greater than the cost of the audio amplifier. A benefit, however, is that if a particular amplifier is not going to be operated from a source of battery power, no concessions in amplifier performance (frequency response, distortion) or cost are required. This is in direct contrast to the previously described "push-pull" amplifier approach, which had low performance and large, expensive transformers, whether or not operation from batteries was required.
It is the object of this invention to circumvent the liabilities of the two previously described methods of obtaining both battery and mains operation from a single amplifier. Specifically, it will be shown that optimum amplifier topologies can be used in battery backed amplifiers despite the limitations imposed by battery operation. Furthermore, this technique allows for the removal of the battery backup circuitry, so that in the instances in which battery operation will not be required, optimum amplifier performance is still provided, without financial burden being placed upon the amplifier.
Another object of the present invention is to provide an audio amplifier system which operates at relatively low current to provide a relatively small transformer and relatively light gauge windings.
A further object of the present invention is to provide an audio amplifier system that is relatively efficient to manufacture.
Another object of the present invention is to provide an audio amplifier system that is simple in construction and efficient in operation.
A still further object of the present invention is to provide an audio amplifier system that may be powered by alternating current but may also utilize battery power if necessary, without placing an additional burden upon the cost of the amplifier.
Other objects and advantages of the present invention will become apparent as the description proceeds.